OT: Re: [lug] cheap 802.11b for linux...

[D. Stimits]

"J. Wayde Allen" wrote:
> 
> On Fri, 22 Mar 2002, Nate Duehr wrote:
> 
> > You can't use RG-6.  It's 75 ohm impedance.
> 
> At least not without suffering mismatch problems or needing to use an
> impedance matching network.
> 
> > RG-213 is double-shielded, which is where it gets its lower loss from,
> 
> Shielding shouldn't directly affect loss.  At least, I can't see why ...?

Think of the skin effect at higher frequencies. Direct current flows
through the center of a conductor, but as frequency goes up, electrons
go to the outer edge of the conductor. If the frequency goes up high
enough, you can actually get competition for space for the electrons
being pushed hard to the outer edge. The more braided edge I would say
the better for the GHz stuff. I suspect that as things go higher and
higher in frequency, the physical shape becomes more important to the
point that a perfect polished pipe might actually be more important than
the braided surface, just because of the radiation patterns.

D. Stimits, stimits at idcomm.com

> 
> > Not all the good stuff is listed there on those pages either.  LMR400
> > is excellent and not listed, and there are others.  Some are more
> > plentiful and actually cheaper in small quantities than others.
> 
> Most definitely true.  I just grabbed those few URL's from a quick web
> search.  There are many others, and certainle some that may be much better
> sources.
> 
> > Look carefully at the sizes also, you'll be surprised at the diameter
> > size differences available.  Typically bigger *usually* means lower loss
> 
> Yes, this is true.
> 
> > because as you move toward "hardline" you move more and more toward air
> > as the dielectric.
> 
> I'm not sure I agree with this.  The field density has to be higher in a
> smaller cable than in a larger one, and I think that this contributes to
> the loss.  Also, the dielectric constant has to be adjusted to maintain
> the 50 ohm cable impedance, so the loss tangent in the dielectric material
> itself may be less.  That is probably what you mean by moving closer to
> air? Anyway, I'll have to think about this a bit more.
> 
> I'd be kind of surprised if you guys are really using hardline.  At least
> what I know as hardline is a solid copper outer conductor, and is a
> probably a bit expensive.  Coaxial lines made from concentric tubes with
> air dielectric are usually called airlines.
> 
> > The other post from Dan did a good job of explaining
> > that, though.
> 
> I have to take a closer look at his post.  Will do that shortly.
> 
> > Waaaaaaaay off topic now, sorry all...
> 
> Yes and no ... this is one of those related, but not specifically Linux
> topics.  Wireless networking is growing and becoming more mainstream.  The
> popularity of the 802.11 hardware is evidence of this.  Also, this touches
> the core technology of the internet itself whether wired or wireless.  In
> either case, you are dealing with RF signals.
> 
> - Wayde
>   (wallen at lug.boulder.co.us)
> 
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